1. Technical Field
The present invention relates to forward converters with synchronous rectification for use in, for example, switching power supplies.
2. Background Art
A forward converter with synchronous rectification in related art is disclosed in Japanese Patent Application No. 3339452. The circuit configuration of the forward converter with synchronous rectification disclosed in Japanese Patent Application No. 3339452 is shown in FIG. 1.
In the circuit shown in FIG. 1, when a primary switching element 2 at the primary side 4a of a transformer 4 is turned on, a rectifier-side synchronous rectifier element 5 at the secondary side is turned on by a voltage generated at a second winding 4b of the transformer 4 while a commutator-side synchronous rectifier element 6 is turned off. If the turning-off of the commutator-side synchronous rectifier element 6 is delayed, a short-circuited path is formed through the two switching elements 5 and 6. Accordingly, the forward converter with synchronous rectification is configured so as to connect a switching element 7 in series to a third winding 4c of the transformer 4 and the forward converter with synchronous rectification is also configured such that the switching element 7 is turned on with a control signal supplied through a pulse transformer 11 at a time when the primary switching element 2 at the primary side is turned on.
With the above configuration, the charge of a parasitic capacitance of the commutator-side synchronous rectifier element 6 is discharged through the switching element 7 immediately before the primary switching element 2 at the primary side is turned on to immediately turn off the commutator-side synchronous rectifier element 6, thus preventing short circuit.
In the configuration described above, the following problems become apparent particularly if the power of the forward converter with synchronous rectification (mainly, the current flowing through the forward converter with synchronous rectification) is high.
First Problem
The first problem is that the waveform of the control voltage for the commutator-side synchronous rectifier element 6 (the waveform of a voltage between the gate and source of the commutator-side synchronous rectifier element 6) is varied and can become a negative voltage.
Specifically, when the primary switching element 2 is turned off, a voltage is generated at the third winding 4c to cause a current to flow toward the gate of the commutator-side synchronous rectifier element 6, and the current flows through the parasitic diode of the switching element 7, which is turned off. Then, the parasitic capacitance between the gate and source of the commutator-side synchronous rectifier element 6 is charged and the gate voltage of the commutator-side synchronous rectifier element 6 increases to turn on the commutator-side synchronous rectifier element 6. As a result, the commutation current flows between the drain and source of the commutator-side synchronous rectifier element 6, instead of through the parasitic diode of the commutator-side synchronous rectifier element 6.
When the primary switching element 2 is turned on, a signal used for turning on the primary switching element 2, output from a control circuit 12, is also applied to the switching element 7 through the pulse transformer 11 to temporarily turn on the switching element 7 slightly before the primary switching element 2 is turned on. The parasitic capacitance between the gate and source of the commutator-side synchronous rectifier element 6 is discharged to turn off the commutator-side synchronous rectifier element 6. However, if the switching element 7 is turned on only for a short time, the parasitic capacitance cannot be sufficiently discharged and the commutator-side synchronous rectifier element 6 may not be turned off completely. Accordingly, the switching element 7 is designed so as to be turned on for a slightly long time in consideration of various variations. However, in this case, the primary switching element 2 is turned on while the switching element 7 is turned on to generate a reverse voltage at the third winding 4c and to positively discharge the charge of the parasitic capacitance of the commutator-side synchronous rectifier element 6 through the switching element 7, which is turned on. As a result, the charge of the parasitic capacitance is not only discharged but also reversely charged to generate a negative gate voltage at the commutator-side synchronous rectifier element 6.
In order to turn off the commutator-side synchronous rectifier element 6, it is sufficient to discharge the charge of the parasitic capacitance and it is not necessary to reversely charge the parasitic capacitance. Accordingly, a current flowing along a path through the parasitic capacitance, the third winding 4c, the switching element 7, and the parasitic capacitance when the parasitic capacitance is reversely charged and a current generated when the reverse charge is discharged during the subsequent cycle are useless, and causes an excessive loss due to the resistance of the current path.
Particularly in a high power direct-current (DC) to DC converter (DC-DC converter), there are cases where multiple synchronous rectifier elements are connected in parallel to each other. In such a case, a number of current paths corresponding to the number of the parallel connections exist to increase the loss by an amount corresponding to the existing current paths and to cause a considerable loss in the entire circuit.
(2) Second Problem
When the parasitic capacitance of the commutator-side synchronous rectifier element 6 is reversely charged, the current flowing through the third winding 4c causes the first winding 4a to generate a current toward the primary switching element 2. Although it seems that no problem is caused because the direction of this current is the same as that of the current flowing when the primary switching element 2 is turned on, the primary switching element 2 is not practically turned on completely (it is going to be turned on) at this stage and the primary switching element 2 has a relatively high resistance. The current is forced to be applied to the primary switching element 2 in this state, thus causing an increase in the loss.
(3) Third Problem
In the configuration shown in FIG. 1, the voltage generated at the third winding 4c is used to control the commutator-side synchronous rectifier element 6. If the load is reduced to shorten the turn-on period of the primary switching element 2, the voltage generated at the third winding 4c is decreased and, therefore, it is not possible to turn on the commutator-side synchronous rectifier element 6. Although an increase in the number of turns of the third winding 4c supposedly resolves this phenomenon, the resistance of the third winding 4c is increased by an amount corresponding to the increase in the number of turns of the third winding 4c, thus increasing the loss.
According to various aspects of the present invention, the following embodiments of a forward converter with synchronous rectification are capable of reducing the losses caused by the foregoing problems.